For example, with an electric power steering device mounted in a vehicle, an electric motor such as a three-phase brushless motor is provided to give steering assist force according to steering torque of a handle to a steering mechanism. As a device for driving this motor, a motor driving device according to a PWM (Pulse Width Modulation) control method has been known.
Generally, the motor driving device according to the PWM control method is provided with an inverter circuit driven by a PWM signal with a predetermined duty. The inverter circuit is formed from a bridge circuit including pairs of upper and lower arms where the number of pairs corresponds to the number of phases and where a switching element is provided to each of the upper and lower arms. A current is supplied from a power source to the motor via the inverter circuit and the motor is driven by the on/off operation of each switching element that is based on the PWM signal.
With such a motor driving device, a general control method is to stop the driving of the motor when an abnormality occurs in any of the phases of the inverter circuit due to a failure of a switching element. However, in the case of an electric power steering device, if the motor is stopped immediately at the time of occurrence of an abnormality, the steering assist force suddenly becomes unobtainable, and operation is adversely affected. Thus, motor driving devices allowing continuation of driving of the motor even when a failure of the switching element of any phase occurs are being proposed (for example, JP 2010-826 A, JP 2005-94873 A, JP 2009-71975 A, and WO 2005/091488 A).
According to JP 2010-826 A, a faulty switching element is identified at the time of occurrence of an energizing failure, and a rotational angle range where the energizing failed phase may be energized via a switching element forming a pair with the faulty switching element is identified. Then, driving of the motor is continued by supplying a sine wave current to each phase within the rotational angle range.
According to JP 2005-94873 A, in the event of an open failure (failure during an off state) of a switching element, vector control with a fixed composite voltage vector is performed even in a region where a faulty switching element is used to thereby continue driving of the motor.
According to JP 2009-71975 A, in the case where a failure of a switching element is detected, driving of the motor is continued by operating remaining operating switching elements in such a way that each of the waveforms of output currents of two phases comes close to a sine wave.
According to WO 2005/091488 A, in the event of an occurrence of an abnormality such as disconnection of one phase, target current values of normal two phases are calculated, and a voltage command value generated based on these target current values is given to an inverter drive circuit, to thereby continue driving of the motor.
For example, regarding a three-phase inverter circuit having six switching elements, if one of upper and lower switching elements of one phase fails, there are conceivable a method of continuing driving of the motor using only the four switching elements of the remaining two phases (hereinafter referred to as a “two-phase method”), and a method of continuing driving of the motor using, in addition to the four switching elements, the one normal switching element of the faulty phase (hereinafter referred to as “quasi three-phase method”). JP 2010-826 A, JP 2005-94873 A, and JP 2009-71975 A are examples of the quasi three-phase method, and WO 2005/091488 A is an example of the two-phase method. In the case of the two-phase method, the entire phase where a failure has occurred is separated, but in the case of the quasi three-phase method, only the faulty switching element is separated. The quasi three-phase method has an advantage that the steering performance is improved because there are less ripple components in the motor current compared to the two-phase method.
FIG. 10 shows an example of a conventional motor driving device used for an electric power steering device. A motor driving device 200 includes an inverter circuit 10, a drive circuit 20, and a controller 30. A motor M is an assist motor for providing steering assist force. A power source Vd is a DC power source to be supplied by a battery mounted in a vehicle.
The inverter circuit 10 is formed from a three-phase bridge circuit including six switching elements Q1 to Q6. The switching elements Q1 to Q6 are formed from FETs (field effect transistors), and include diodes D1 to D6, respectively. These diodes D1 to D6 are parasitic diodes between drains and sources of the FETs, and are connected in parallel to the switching elements Q1 to Q6, in opposite direction with respect to the power source Vd (positive terminal). Each of the connection points between the upper switching elements Q1, Q3, and Q5 and the lower switching elements Q2, Q4, and Q6 is connected to the motor M via an electrical path. A current sensing resistor Rs for sensing a current flowing through the motor M is provided between a ground G and the switching elements Q2, Q4, and Q6.
The controller 30 calculates the duty of the PWM signal for driving the switching elements Q1 to Q6 based on a difference between the value of a motor current sensed by the current sensing resistor Rs and a target current value calculated from a steering torque value detected by a torque sensor not shown. The drive circuit 20 generates six types of PWM signals based on the value of duty given by the controller 30, and applies each PWM signal to each of the gates of the switching elements Q1 to Q6. The inverter circuit 10 operates by the switching elements Q1 to Q6 being switched on/off by the PWM signals, and a current is supplied to the motor M from the power source Vd via the inverter circuit 10.
FIG. 11 is a time chart showing the operation of the inverter circuit 10. (a) and (b) show a PWM signal given to the switching element Q1 of a phase A upper stage and an operation of the switching element Q1, (c) and (d) show a PWM signal given to the switching element Q2 of a phase A lower stage and an operation of the switching element Q2, (e) and (f) show a PWM signal given to the switching element Q5 of a phase C upper stage and an operation of the switching element Q5, (g) and (h) show a PWM signal given to the switching element Q6 of a phase C lower stage and an operation of the switching element Q6, (i) shows motor terminal voltage of the phase A, and (j) shows motor terminal voltage of the phase C. With respect to the PWM signal, “H” indicates a high level, and “L” indicates a low level. Moreover, to simplify the description, a phase B is omitted from the drawing.
In FIG. 11, a section T1 between time t1 and t2 is a power-running section. In this section T1, the switching elements Q1 and Q6 are switched to an on state as shown in (b) and (h), and the switching elements Q2 and Q5 are switched to an off state as shown in (d) and (f), and thus, a current flows from the power source Vd to the motor M by a path shown by the dashed line in FIG. 12A.
A section T2 between time t2 and t3 is a lower-stage regeneration section. In this section T2, the switching elements Q2 and Q6 are switched to the on state as shown in (d) and (h), and the switching elements Q1 and Q5 are switched to the off state as shown in (b) and (f), and thus, a regenerative current based on discharge of energy stored in an inductance of the motor M flows through a path shown by the dashed line in FIG. 12B.
A section T3 between time t3 and t4 is again a power-running section, and the switching elements Q1 and Q6 are switched to the on state as shown in (b) and (h), and the switching elements Q2 and Q5 are switched to the off state as shown in (d) and (f), and thus, a current flows from the power source Vd to the motor M by a path shown by the dashed line in FIG. 12C (which is the same as that in FIG. 12A).
A section T4 between time t4 and t5 is an upper-stage regeneration section. In this section T4, the switching elements Q1 and Q5 are switched to the on state as shown in (b) and (f), and the switching elements Q2 and Q6 are switched to the off state as shown in (d) and (h), and thus, a regenerative current based on discharge of energy stored in the inductance of the motor M flows through a path shown by the dashed line in FIG. 12D.
A section T5 between time t5 and t6 is again a power-running section, and the same patterns as those of the sections T1 to T4 are repeated thereafter.
A failure where one of the switching elements Q1 to Q6 of the inverter circuit 10 as described above is fixed to the off (non-conducting) state and would not be switched on (conducting) sometimes occurs. This failure is referred to as an “off failure” in this specification (the “open failure” in JP 2005-94873 A is used synonymously).
In the case where six switching elements Q1 to Q6 to which diodes D1 to D6 are connected in parallel are used, as in FIG. 10, there are two types of off failures. One is a failure where both the switching element and the diode would not conduct (hereinafter referred to as a “complete off failure”). This is a failure that occurs when disconnection occurs on the drain side or the source side of the FET, and both the switching element and the diode are cut off, for example. The other is a failure where the diode is normal, and only the switching element would not conduct (hereinafter referred to as an “incomplete off failure”). This is a failure that occurs when the gate of the FET is shunted to the ground, or the FET itself is broken, for example.
FIG. 13 shows a state at the time of upper-stage regeneration where there is an occurrence of the “complete off failure” at the phase C upper stage of the inverter circuit 10. At this time, the switching element Q5 is in the abnormal (non-conducting) state, and a regenerative current cannot flow through the inverter circuit 10 via the switching element Q5. Since the diode D5 is also in the abnormal (non-conducting) state, the regenerative current cannot flow through the inverter circuit 10 via the diode D5 either.
FIG. 14 shows a state at the time of upper-stage regeneration where there is an occurrence of the “incomplete off failure” at the phase C upper stage of the inverter circuit 10. At this time, the switching element Q5 is in the abnormal (non-conducting) state, and a regenerative current cannot flow through the inverter circuit 10 via the switching element Q5. However, the diode D5 is normal, and the regenerative current flows through the inverter circuit 10 via the diode D5 by a path as shown by the dashed line.
In a state where there is an occurrence of a complete off failure at the phase C upper stage, as in FIG. 13, surge voltage in the positive direction occurs in the motor terminal voltage of the phase C (the voltage at the connection point of the switching elements Q5, Q6) at the timing of switching from the power-running state to the upper-stage regeneration state (time t4). This is because the energy stored in the inductance of the motor M appears as the surge voltage without being absorbed as the regenerative current at the moment of switching off of the switching element Q6 (normal) of the phase C lower stage at time t4. When the voltage value of this surge voltage exceeds the withstand voltage of the switching element Q6, the switching element Q6 is destroyed.
Accordingly, even if there is an attempt to perform driving by the quasi three-phase method described above by switching on/off the normal switching element Q6 of the phase C lower stage in a state where there is a complete off failure at the phase C upper stage, driving by the quasi three-phase method is impossible when the switching element Q6 is destroyed. In this case, switching to driving by the two-phase method has to be performed, but conventionally, there has been no means for determining which of the quasi three-phase method and the two-phase method should be adopted.
On the other hand, in a case where the phase C upper stage is in the state of an incomplete off failure, a regenerative current flows at the time of upper-stage regeneration, and no surge voltage occurs in the motor terminal voltage of the phase C. Therefore, driving by the two-phase method by completely separating the phase C without switching on/off the switching element Q6 of the phase C lower stage is not only pointless, but also undesirable because the ripple components in the motor current would be increased compared to the quasi three-phase method.
FIG. 16 shows a state at the time of lower-stage regeneration where there is an occurrence of the “complete off failure” at the phase A lower stage of the inverter circuit 10. At this time, since the switching element Q2 is in an abnormal (non-conducting) state, a regenerative current cannot flow through the inverter circuit 10 via the switching element Q2. Since the diode D2 is also in the abnormal (non-conducting) state, the regenerative current cannot flow through the inverter circuit 10 via the diode D2.
FIG. 17 shows a state at the time of lower-stage regeneration where there is an occurrence of the “incomplete off failure” at the phase A lower stage of the inverter circuit 10. At this time, since the switching element Q2 is in an abnormal (non-conducting) state, a regenerative current cannot flow through the inverter circuit 10 via the switching element Q2. However, the diode D2 is normal, and the regenerative current flows through the inverter circuit 10 via the diode D2 by a path as shown by the dashed line.
In a state where there is an occurrence of a complete off failure at the phase A lower stage, as in FIG. 16, surge voltage in the negative direction occurs in the motor terminal voltage of the phase A (the voltage at the connection point of the switching elements Q1, Q2) at the timing of switching from the power-running state to the lower-stage regeneration state (time t2). This is because the energy stored in the inductance of the motor M appears as the surge voltage without being absorbed as the regenerative current at the moment of switching off of the switching element Q1 (normal) of the phase A upper stage at time t2. The source potential of the switching element Q1 is lowered by this surge voltage in the negative direction, and as a result, the switching element Q1 falls into a semi-on state (an intermediate state between on and off). Thus, a large current based on the surge voltage flows to the power source Vd side via the switching element Q1, and when power loss (heating) due to this large current becomes excessive, the switching element Q1 is destroyed.
Accordingly, even if there is an attempt to perform driving by the quasi three-phase method described above by switching on/off the normal switching element Q1 of the phase A upper stage in a state where there is a complete off failure at the phase A lower stage, driving by the quasi three-phase method is impossible when the switching element Q1 is destroyed. Also in this case, switching to driving by the two-phase method has to be performed, but conventionally, there has been no means for determining which of the quasi three-phase method and the two-phase method should be adopted.
On the other hand, in a case where the phase A lower stage is in the state of an incomplete off failure, a regenerative current flows at the time of lower-stage regeneration, and no surge voltage occurs in the motor terminal voltage of the phase A. Therefore, driving by the two-phase method by completely separating the phase A without switching on/off the switching element Q1 of the phase A upper stage is not only pointless, but also undesirable because the ripple components in the motor current would be increased compared to the quasi three-phase method.